As shown in FIG. 1, a flyback power converter for power supply applications includes a transformer Tx connected between a primary circuit 10 and a secondary circuit 12, the primary circuit 10 uses a controller 14 to switch a power switch 16 to control the primary current of the transformer Tx, and when the power switch 16 is turned off, the transformer Tx delivers energy to the secondary side to charge a capacitor C to supply a load current Io. In the secondary circuit 12, an output feedback circuit 18, for example, the photo-coupler shown in FIG. 1, is connected to the output Vo for output feedback, a shunt regulator 20 has a cathode K connected to the photo-coupler 18 to control the current Id of the light-emitting diode (LED) in the photo-coupler 18, and resistors R1 and R2 are connected in series between the output Vo and a ground of the flyback power converter. The resistor R2 has an end connected to the reference end R of the shunt regulator 20, thereby scaling the output voltage of the flyback power converter toVo=VR×[(R1+R2)/R2]=VR×[1+(R1/R2)],  [Eq-1]where VR is the reference voltage provided by the shunt regulator 20. Assuming that the resistances between the output Vo of the flyback power converter and the two ends of the LED in the photo-coupler 18 are R3 and R4, respectively, the LED current flowing from the output Vo to the photo-coupler 18 will beId=[Vo−(VK+1.1)]/R3,  [Eq-2]the bias current flowing from the output Vo to the bias point K will beIbias=(Vo−VK)/R4,  [Eq-3]and the voltage divider current flowing from the output Vo to the resistors R1 and R2 will beIr=Vo/(R1+R2).  [Eq-4]When the system enters a standby mode, even the load current Io decreases to very low, the output voltage Vo of the flyback power converter still remains unchanged and as a result, as shown by the equations Eq-2 to Eq-4, the currents Id, Ibias, and Ir in the standby mode are the same as in normal operation. In other words, the photo-coupler 18, the shunt regulator 20, and the resistors R1, R2, R3, R4 consume considerable power even in standby mode, thus incurring excessive loss of standby power of the flyback power converter.